identity mapped paging (Vaibhav Jain)

[Liu Zhiyou]

Hi,

I am not very sure what you guys are talking about. I guess we are
discussing the moment enabling paging.

On 2012-4-15 10:49, Vladimir Murzin wrote:
>  On Fri, Apr 13, 2012 at 02:15:20AM -0700, Vaibhav Jain wrote:
>>>>  I am not clear about the use of identity mapped paging while paging is
>>>>  being enabled by the operating system. Also I don't understand at what
>>>>  point are the
>>>>  identity mappings no longer useful.According to this article
>>>>  http://geezer.osdevbrasil.net/osd/mem/index.htm#identity  - "The page
>>>>  table
>>>>  entries used to identity-map kernel memory can be deleted once paging
>>>>  and
>>>>  virtual addresses are enabled." Can somebody please explain?
>>>>
>>>  Identity mapping is when VA(Virt Address)=PA(Physical address).
>>>
>>>  So basically when you set up your page tables you need to make sure they
>>>  map identically. This is very easily done if you consider each 4KB block as
>>>  a page beginning from location 0 upto whatever you've found to be the
>>>  highest memory available either thru BIOS or GRUB.
>>>
>>>  Remember that while setting up your PTEs and PDE every address is a
>>>  physical one. So if you thought that your kernel would be linked initially
>>>  to a higher VA since you would remap it to a lower memory physically then
>>>  that would be WRONG!. Without PTEs and PDEs installed don't do that!.
>>>
>>>  Why would you want it? Well for a simple reason, when your kernel starts
>>>  to boot there's no translator,(No PTEs/PDEs and the Paging Enabled bit of
>>>  processor is also cleared AFAIK just after the BIOS is done), yet since
>>>  you've not enabled your processor for that but you'll be doing that in a
>>>  moment.
>>>
>>>  So let's say you made your kernel to be linked to higher VA like 3Gigs.
>>>  Now the addresses would be generated beginning 3Gigs however you still
>>>  don't have the Page tables installed since your kernel just started. So in
>>>  that case the address is the physical address. And if you've not loaded
>>>  your kernel beginning 3Gigs then it would definitely come crashing down.
>>>
>>>  To avoid the crash in case you made your kernel to link to higher half of
>>>  the memory, you can use GDT trick since segmentation is always on and you
>>>  can make the overflow of the address addition to translate to a lower
>>>  physical memory even if paging is not enabled yet. Thus it is possible to
>>>  load the kernel at lower memory addresses while the linkage would be for
>>>  higher VMA. And once your PTEs/PGD are enabled then you can use those
>>>  instead of the GDT trick.
>>>
>>>  Here's a link to thathttp://wiki.osdev.org/Higher_Half_With_GDT
>>>
>>>>  Thanks
>>>>  Vaibhav Jain
>>  Hi,
>>
>>  Thanks for replying but I am still confused. I continued reading about this
>>  thing and what
>>  I have understood is the following :
>>  After the kernel executes the instruction to enable paging the instruction
>>  pointer will contain the
>>  address of the next instruction which will now be treated as a virtual
>>  address. So for the next instruction to be executed
>>  the page table should map this address to itself.
In this moment, the segment transition is still on, and physical address
is got from

Virtual Address ------------------>  Linear Address
----------------------->  Physical Address.
                 Segment                   Paging

Without identical mapping in LA[0]. PA = VA - 2* (kernel base address). With
identical mapping in LA[0], PA = VA - (kernel base address). It is just
what we
want. And with this mapping, we will disable the Segment transition by
setting
new segment offset to 0. After that, the PDE and PTE we seted for linear
address works and we don't need the identical mapping anymore.

Hope it heles.
>>  Please correct me if I am wrong.
>>  I am confused by the point about linking  the kernel to higher address.
>>  Could you please put that in a step by step manner
>>  to make it clear what  happens before paging is enabled and what happens
>>  after that.
>>  Also, please explain at what point during the execution of kernel code are
>>  the identity-mapped addresses no longer useful ?
>>
>>
>>
>>
>>  Thanks
>>  Vaibhav
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>  Hi
>
>  May be notes in [1] make it a bit clear ;)
>
>  [1]http://pankaj-techstuff.blogspot.com/2007/12/initialization-of-arm-mmu-in-linux.html
>
>  Best wishes
>  Vladimir Murzin
>
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Regards,
Lewis Lau